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mammals

...pair off, and mate in seclusion. The male leaves the female soon after mating and plays no role in raising the young. Gestation periods vary, the fertilized egg remaining dormant in the uterus (
delayed implantation), which ensures the birth of young while the female is in the winter den and guarantees that the cubs will emerge from the den in the spring, when food is abundant. Ursids breed...

...July and August, develop for a few days, then lie dormant in the uterus, being implanted in January. Birth occurs in March. Of the total gestation period of 250 days, growth occurs during only 50. Delayed implantation also occurs in mice and other small rodents that become pregnant while they are still suckling a litter.

...and many species possess reproductive peculiarities as adaptations to their environments. Induced ovulation, for instance, allows females to release egg cells during or shortly after copulation. Delayed implantation of the fertilized egg in the wall of the uterus is another phenomenon that allows births to occur when resources are abundant. This phenomenon is most prominent in species living...